A good fanfic of them should have their distinct personalities shine through. Rainbow Dash's confidence and athleticism, and Pinkie Pie's wackiness and friendliness. If it can capture that essence, it's a great start.
In a Rainbow Dash and Pinkie Pie fanfic, it could be centered around their shared experiences. Maybe they enter a competition together. Since Rainbow Dash is competitive and Pinkie Pie loves to be involved in things, this could lead to some really interesting scenarios. The story might also explore how they deal with their differences. For example, Rainbow Dash might get frustrated with Pinkie Pie's chaos at times, but then realize that it's part of what makes her special. And of course, there will likely be lots of colorful descriptions of Equestria and the other ponies they interact with.
Friendship is a very common theme. Since they are both characters from the MLP universe, fanfics often explore how their friendship develops and grows. They might start as acquaintances and become best friends, or they might have some bumps in their friendship that they need to overcome.
Adventure is also a popular theme. They might go on a quest together, like looking for a lost treasure or saving Ponyville from a mysterious threat. This allows for a lot of action and character development. In these types of fanfics, Rainbow Dash's bravery and Pinkie Pie's spontaneity often play important roles.
Character development is key. In these fanfics, we often see Rainbow Dash becoming more in - touch with her softer side thanks to Pinkie Pie, and Pinkie Pie might become more responsible with Rainbow Dash's influence. Also, the fanfic should capture their unique personalities well. Rainbow Dash's competitiveness and Pinkie Pie's wackiness should be on full display.
Another interesting one is 'When Pinkie Met Dash'. This fanfic delves into their first encounter and how it led to the development of their relationship over time. It has a lot of cute and humorous moments that really capture the essence of their personalities.
There's also 'Rainbow Dash and Pinkie Pie: Unlikely Allies'. In this fanfic, they start off on different paths but end up coming together to solve a big problem in Ponyville. It delves deep into their personalities and how they complement each other. It's quite long and detailed, which is great for those who like to really get into the story. The descriptions of their emotions and the situations they face are very vivid.
There are quite a few 'rainbow dash x pinkie pie fanfic' out there. Some focus on how they support each other during tough times in Ponyville. Like when there was a big storm and Pinkie Pie used her cheerfulness to lift Rainbow Dash's spirits, and Rainbow Dash in turn protected Pinkie Pie from the strong winds.
A 'mlp pinkie pie sad fanfic' might explore the emotional depth of Pinkie Pie in a rather somber way. It could involve situations where Pinkie Pie faces disappointment, loss of friends, or failure in her party - planning endeavors. This type of fanfic gives a different perspective on her usually cheerful character.
There's also a fanfic where Pinkie Pie goes on a journey to find a special ingredient for a super - delicious cake. In this fanfic, we see her determination and her ability to make friends along the way. It shows how Pinkie Pie's positive attitude can get her through any situation. The interactions between Pinkie and the new characters she meets are really engaging.